In a battery pack (battery module) configured of a plurality of secondary battery cells connected in series, in some cases, states of charge (SOC) may be different between the respective secondary battery cells due to unbalanced temperature distribution and unbalanced self-discharge. Such a state is called “disruption of cell balance.” In the case where the disruption of cell balance occurs, for example, a state that while one secondary battery cell is in a state of full charge, another secondary battery cell is not sufficiently charged occurs. When charge is continued in such a state, the secondary battery cell in a state of full charge is overcharged, and liquid leakage, heat generation, and/or the like may occur therein. Therefore, control is executed so that the secondary battery cell in a state of full charge is not charged any further. However, in this case, the capacity of the secondary battery cell not sufficiently charged yet is not used maximally. On the other hand, at the time of discharge, when the capacity of the secondary battery cell not sufficiently charged yet is used up, part of the capacity of the secondary battery cell that has been in a state of full charge remains. Therefore, the capacity of the secondary battery cell that has been in a state of full charge is not used up as well.
To solve the foregoing issue, in the technology disclosed in Japanese Unexamined Patent Application Publication No. 2009-261168, at the time other than discharge, connection of respective secondary battery cells is switched to parallel connection to prevent respective SOCs from being unbalanced. Further, in the technology disclosed in Japanese Unexamined Patent Application Publication No. 2008-278635 (JP2008-278635A), secondary battery cells are connected in series at the time of discharge, and the secondary battery cells are firstly connected in series and are subsequently switched to be connected in parallel at the time of charge. Further, in the technology disclosed in JP2008-278635A, in order to prevent charge time from being increased at the time of parallel connection, series connection is maintained at the time of CC charge, and the series connection is switched to parallel connection when the CC charge is shifted to CV charge.